Petroleum distillates comprise fractions which boil within the range of about 221.degree. C. to 485.degree. C., obtained by the atmospheric or vacuum distillation of crude oil. Light gas oil (221.degree. C.-343.degree. C.) after hydrorefining and/or fractionation may be used as fuel in diesel engines, jet planes or for home heating oil. Heavy gas oils (343.degree. C.-565.degree. C.) may be used as boiler fuels or else treated by catalytic cracking to produce high octane motor gasoline. In those instances where petroleum distillate fractions are utilized as fuels, it becomes necessary to meet fuel specifications, particularly with respect to pour point and ignition quality.
Ignition quality of petroleum distillates such as diesel fuel is related to the delay encountered between injection and combustion of the fuel in a diesel engine. The ignition delay period is an important factor in diesel engine combustion. A long delay period at high engine loads results in a rapid increase in pressure when the fuel starts to burn. The rate of pressure rise may become so rapid at high engine loads that knock, and/or rough engine operation, occurs. By way of explanation, it is generally believed that a long delay period allows more time for certain chemical reactions to take place in the combustion chamber before ignition occurs. These reactions result in products that burn very rapidly, causing excessively rapid pressure rise. With a short delay period, ignition apparently occurs before these reactions have proceeded far enough to cause too rapid burning. Also, with a cold engine and low intake-air temperatures, too long a delay period produces misfiring and uneven or incomplete combustion, with consequent white smoke and loss of power. Although the ignition delay time is influenced by engine operating conditions, it is known that this time for straight-run distillates depend primarily on the hydrocarbon composition of the fuel and, to a lesser extent, on its volatility characteristics. A fuel that exhibits a long delay period is said to have poor ignition quality.
The ignition quality of diesel fuel may be quantified by various methods including, for example, determining the temperature (T.sub.20) needed to produce a twenty second ignition delay; and by its Cetane Number, as determined by ASTM. These two measures of burning quality (with the possible exception for n-alkanes noted below) appear to correlate well. U.S. Pat. No. 4,549,815 to Venkat et al. describes an apparatus and method for measuring the cetane quality of a distillate fuel by measuring the temperature required for a twenty-second ignition delay, hereinafter referred to as the t.sub.20 ignition temperature. In accordance with that invention, the ignition delay of distillate fuel is measured by apparatus which includes a block having an ignition cavity. The block is heated to an elevated temperature above the ignition temperature of the fuel and then allowed to cool slowly. As it cools, samples of fuel are injected into the ignition cavity at times which are controlled by a digital computer. A pressure transducer and a thermocouple measure the pressure and temperature, respectively, in the cavity. For each injected sample, the digital computer measures ignition delay as the time between injection of a sample and ignition as indicated by a peak in measured cavity pressure or cavity temperature. The ignition delay is recorded as a function of the cavity temperature prior to fuel injection.
Using the temperature required for a given ignition delay, the cetane rating of distillate fuels may be estimated from a calibration curve established by comparing unit data with results from the ASTM cetane number test. It has been found that the ignition temperatures of the tested distillate fuels fall n a smooth correlation curve which can be used to provide cetane number estimates for unknown fuels. These estimates are in excellent agreement with observed ASTM values. The entire content of U.S. Pat. No. 4,549,815 is incorporated herein by reference as if fully set forth. In general, all references made herein to ignition quality, unless explicitly stated to be otherwise, are to be understood as referring to that quality determined by measurement of the t.sub.20 temperature as described in U.S. Pat. No. 4,549,815 or equivalent, and to the estimate of cetane number derivable therefrom. It is noted that n-alkanes have been reported to have longer ignition time (i.e. higher t.sub.20 temperature and lower calculated cetane value) than conventional diesel fuel of similar cetane number. (See M. Fortnagel et al.; Proceedings of American Petroleum Institute, Refining Dept., 61, pp 43-53, 1982.) However, since the present invention is not concerned with n-alkane feeds, this deviation is deemed to be not relevant.
In general, aromatic hydrocarbons are reputed to be of low ignition quality, while paraffins are believed to have high ignition quality. Thus, it is apparent that the base stocks used in blending to make diesel fuels are important in determining ignition quality. The refiner is constantly faced with the problem of blending stocks to achieve adequate ignition quality without sacrificing other necessary characteristics, such as pour point and volatility.
In the past, it has been known to upgrade the ignition quality of low quality cracked petroleum distillate such as FCC cycle oils by adding a blending component such as a straight-run gas oil fractions. However, the amount of FCC cycle oil, usually of high aromatic content, that may be used as a blending stock is severely limited because its inclusion in any substantial quantity causes excessive deterioration of the ignition quality of the blend.
That aromatic hydrocarbons can be alkylated by olefins in the presence of an acid catalyst is known. Such catalysts include conventional Lewis acids such as aluminum chloride and a variety of large pore size crystalline zeolites. In general, with propylene or higher olefins, the reaction proceeds in accordance with Markownikoff's rule, i.e. the aromatic moiety reacts with the olefin at the carbon atom having the least hydrogen, with no substantial formation of n-alkyl aromatic product.
U.S. Pat. No. 4,021,331 to Ciric broadly describes the catalytic conversion of organic compounds by zeolite ZSM-20. The patent includes a description of the preparation and properties of the zeolite ZSM-20. U.S. Pat. No. 4,570,027 to Boucher et al. describes a process for alkylating aromatic hydrocarbons with olefins using partially collapsed zeolite catalyst. U.S. Pat No. 4,301,316 to Young describes the synthesis of phenyldodecane by reaction of 1-dodecene and benzene, catalyzed by mazzite, Zeolite Beta, ZSM-20, ZSM-38 and isotypes thereof. The foregoing patents are incorporated herein by reference for background purposes.
It is an object of the present invention to provide a process for upgrading the quality of FCC cycle oil as blending stock for diesel fuel and heating oil. It is a further object to provide a catalytic process for alkylating FCC cycle oil with olefins wherein catalyst aging is reduced. I is a still further object to provide a process for alkylating FCC cycle oil with linear olefins thereby achieving much enhanced ignition quality. These and other objectives will become evident on reading this entire specification including the appended claims.